Hollow fan or compressor blades are used to provide stiff lightweight blades. Titanium is used for these blades because of the high strength provided with low weight. This material selection is made despite the many problems in the forming of titanium structures.
One method of forming such a titanium blade is shown in U.S. Pat. No. 5,063,662 issued Nov. 12, 1991 to Porter et al. There, two blade halves are machined and diffusion bonded together. A gas injection tube is simultaneously bonded between the two halves. The blade is later twisted and formed at high temperature, with gas pressure introduced inside the blade.
These gas tubes used in the manufacturing of hollow blades introduce gas pressure inside the part to remove any skin buckles or irregularities during processing, particularly final forming. The tubes are placed in slots machined into detail halves and bonded to the part as part of the bond cycle. It is important that a good seal exist at the interface between the gas tube and the bonded blade because a leak will cause internal-contamination of the part during subsequent operations.
During the forming operation the material is at a temperature such that the internal gas pressure will cause the material to deform. With the gas tube of the same material as the blade, the tube will deform and therefore be unable to contain the pressure. Accordingly early parts used a tube made of stainless steel for the portion outside the blade with titanium forming the portion inside the blade. Since these materials cannot easily be joined, a tantalum interface was located between the two materials.
The titanium end was placed in the slot for bonding, with the stainless steel end attached to the gas supply line and exposed to the environment. The stainless steel would withstand the applied gas pressure without deformation in the final formation. However the titanium interface would oxidize and become brittle causing failure. Furthermore the titanium tube within the blade detail would sometimes either be crushed closed during the diffusion bonding portion, or insufficiently resist the pressure of the two halves resulting in a poor bond.
Stainless steel tubes plated with copper nickel were then used. The copper nickel plating would act as a braze material at bond temperatures allowing the tubes to be brazed in position and provide a good seal. The problem with this braze is that the plating material, copper, would migrate into the bond plane of the part, creating an unacceptable bond in that local area.
The need still exists for a gas tube which will form a good bond within the titanium blade without contaminating the interface, and which will withstand the applied gas pressure for formation of the final blade at temperature.